Process for effecting mass transfer between a liquid phase and a gaseous phase

ABSTRACT

A process for effecting mass transfer between a liquid phase and a gaseous phase in a filled-type column comprising an external shell which accommodates at least one filler-containing basket wherethrough the phases are caused to flow in countercurrent relationship, advantageously comprises the step of feeding the gaseous phase to the at least one basket through a gas-permeable surface thereof which is larger than the basket cross-section, preferably in a prevailing radial flow direction.

FIELD OF THE INVENTION

[0001] This invention relates to a process for effecting mass transferbetween a liquid phase and a gaseous phase in a filled-type columnwherethrough said phases are caused to flow in a countercurrentrelationship.

[0002] In the description given below and in the following claims, theterm: “filled-type column”, is understood to mean—in general—anapparatus having, disposed on its interior, a plurality of variouslyshaped and sized elements (filler) at whose surfaces a liquid phase anda gaseous phase are caused to contact each other in order to effect masstransfer; apparatus of this type are widely employed in chemical plants,e.g. for decomposing, absorbing, distilling, and scrubbing chemicals.

[0003] The invention also relates to a filled-type column forimplementing the above process, and to a method of retrofitting apre-existing column for conversion into a filled-type column accordingto the invention.

[0004] In the description given below and in the following claims, theterm: “retrofitting”, is understood to mean the in-situ modification ofa pre-existing column of the filled or plates type to improve itsperformance and, for example, to increase its capacity and/or itsefficiency in mass transfer between the liquid phase and the gas phase,as well as to reduce its energy consumption.

[0005] As is known, in the field there is increasingly felt the need ofproviding processes easy to implement, which can effect mass transferbetween a liquid phase and a gaseous phase in a simple and efficientway, at low investment and operating costs and with a low energyconsumption.

PRIOR ART

[0006] In order to meet the above requirement, processes for effectingmass transfer have been proposed in the field wherein a liquid phase anda gaseous phase are caused to flow through a filled-type column in adownward and an upward (substantially axial) direction, respectively.

[0007] While being simple to implement, such prior processes are besetwith problems arising mainly from the large pressure drop experienced bythe gas phase in going through the filler.

[0008] The filler is, in fact, disposed inside substantially cylindricalcolumns having a large height-to-diameter ratio intended to ensure aninterphase contact of sufficient duration for the phases to enhance themass transfer.

[0009] Consequently, in flowing through the filler, the gas phaseundergoes a significant reduction in pressure (pressure drop), whichplaces limitations on the amount of gas that can be fed into the column,thus curtailing the column capacity.

[0010] By reason of this pressure drop, the flow rate of the gas phasefed into the column must be held below predetermined values, theovertaking of which would result in the appearance of an undesirable“flooding” phenomenon, whereby the column becomes flooded with liquidphase which is held back in its downward movement by frictional dragfrom the gas phase. It will be appreciated that in this condition thecolumn would no longer be operable, due to the mass transfer dropping tovirtually nil.

[0011] In other words, the large pressure drop of the gaseous phase ingoing through the filler is a limiting factor of the gas phase flow ratethrough the column designed to implement the above prior art processes,and hinders an effective mass transfer between the phases.

[0012] To counteract the pressure drop in the gaseous phase, and thus toincrease the capacity of filled-type columns according to the prior art,some special fillers having a high void ratio have been proposed,wherethrough the gaseous phase stream undergoes no large pressure drops.

[0013] While such fillers allow the flow rate of the gas phase being fedinto the column to be increased somewhat, they have been unsuccessful inproviding a fully satisfactory mass transfer between the phases, becausethe pressure drop of the gas phase through the column is still quitesignificant, as are the operational constraints due to theaforementioned flooding phenomenon.

[0014] Columns of the so-called plates type, i.e. provided with aplurality of perforated plates fitted horizontally inside the column,have been proposed alternatively to the filled-type columns.

[0015] In this case, the process for effecting mass transfer between theliquid and gaseous phases comprises mixing the phases together in theplates, the plates being typically designed to enhance the mutualcontact of the phases.

[0016] Plates-type columns have been found suitable for low liquidflow-rate applications, but are liable to become flooded, disallowingany further effective mass transfer between the liquid and gas phases.

[0017] It is on account of the above disadvantages that prior artprocesses for effecting mass transfer between a liquid phase and agaseous phase have provided unsatisfactory performance heretofore, bothin terms of overall efficiency of the transfer between the phases,energy consumption, and operating and investment costs of the columnsused to implement such processes (which are, as said before, beset withseveral limitations).

[0018] All this, despite this technology having been utilized in avariety of chemical applications for years and the aforementioned needbeing increasingly felt in the field.

SUMMARY OF THE INVENTION

[0019] The problem underlying the present invention is that of providinga process for effecting mass transfer between a liquid phase and agaseous phase, which process affords a highly efficient transfer betweenthe phases in a simple and effective way, at low investment andoperating costs and with low energy consumption.

[0020] This problem is solved, according to the invention, by a processas indicated above for effecting mass transfer between a liquid phaseand a gaseous phase within a filled-type column which comprises anexternal shell accommodating at least one filler-containing basketwherethrough said phases are caused to flow in countercurrent, whichprocess is characterized in that it comprises the step of feeding saidgaseous phase to said at least one basket through a gas-permeable basketsurface which is larger than the basket cross-section.

[0021] In this way, by causing the gaseous phase to flow through apermeation surface of said at least one basket which is advantageouslymade larger than the basket cross-section, a corresponding reduction isobtained in the pressure drop of said phase flowing through the filler,which allows the gas flow rate to be increased and—at the same time—tooperate at slower velocities than prior art processes, therebysignificantly enhancing the mass transfer between the phases.

[0022] The benefits in terms of improved efficiency of mass transfer arethe more substantial, the larger the permeation surface presented to thegaseous phase.

[0023] In this respect, the above problem is solved, in a particulareffective way, by a process for effecting mass transfer between a liquidphase and a gaseous phase within a filled-type column which comprises anexternal shell accommodating at least one filler-containing basket whosecross-section is smaller than the cross-section of said shell, saidprocess comprising the steps of:

[0024] feeding said liquid phase and gaseous phase into said filled-typecolumn;

[0025] causing the liquid phase to flow through said at least one basketin a substantially axial direction;

[0026] causing the gaseous phase to flow through said at least onebasket in a prevailing radial direction;

[0027] extracting said liquid phase and gaseous phase from saidfilled-type column.

[0028] With the process of this invention, and particularly by causingthe gaseous phase to flow through the filler in a prevailing radialdirection—rather than in an axial direction as taught by prior artprocesses—the permeation or passage surface can be drastically expandedto positively enhance in a simple and effective way the mass transferbetween the phases and, hence, to significantly increase the capacity ofthe column designed to implement such a process over a column ofcomparable size operating according the aforementioned prior artprocesses.

[0029] Stated otherwise, for a given capacity, the column implementingthe process of this invention can by made substantially smaller than aprior art column.

[0030] This arises mainly from that the pressure drop undergone by thegas phase in flowing radially through the filler can be consideredtrivial compared to the pressure drop involved in going through thefiller in the axial direction, so that the process of this invention canadvantageously be carried out at higher gas flow rates than in the priorart, before such undesirable phenomena as flooding occur, therebyenhancing the mass transfer between the liquid and the gaseous phase.

[0031] In particular, the process of this invention can virtuallyeliminate—in an extremely simple and effective way—the constraints ofpressure drop and limited flow-rate of the gas phase through the column,allowing to operate at desired volumes and velocities of the gas andliquid flows for optimum utilization of the exchange surface area of thefiller for effecting mass transfer.

[0032] Advantageously, thanks to this process, a highly efficient masstransfer between the phases can now be achieved using a filled-typecolumn which performs most efficiently, and involves low investment andoperating costs and at low energy consumption.

[0033] The fact that the mass transfer between the phases can besignificantly improved by flowing the gas phase in a prevailing radialdirection through the filler, is in sharp contradiction with theconstant prior art teaching that a crossflow of the gaseous phaserelative to the liquid phase is less advantageous, from the standpointof the mass transfer, than an axial flow of the phases in truecountercurrent relationship.

[0034] In addition, since the conditions and the chemical/physicalprocesses which are responsible for the mass transfer between the phasescan vary substantially with the flow type, the benefits of the processaccording to the present invention were not at all predictable a priori.

[0035] It was only the research work carried out by the Applicant thatunexpectedly brought out how, by having the gaseous phase flow in aprevailing radial direction, in countercurrent to the substantiallyaxial flow of the liquid phase, the driving force responsible for themass transfer between the phases could act much more efficiently than ina true countercurrent flow, and thus the effectiveness and intensity ofthe mass transfer could be advantageously augmented.

[0036] Particularly satisfactory results have been obtained by causingthe gas phase to flow through said at least one basket in substantiallyradial, substantially axial-radial, or substantially crosswisedirections.

[0037] Preferably, the prevailing radial flow of the gaseous phasethrough said at least one basket is of an either centrifugal, orcentripetal, or alternating centrifugal and centripetal type.

[0038] In a particularly advantageous embodiment of the invention, theliquid phase and the gas phase are caused to flow in said at least onebasket through a plurality of contiguously superimposed zones, with theliquid phase and the gas phase being fed to an upper zone and a lowerzone, respectively, of said at least one basket, and extracted from alower zone and an upper zone thereof, respectively.

[0039] By dividing the filler into a plurality of zones to be traversedby the gas phase in a prevailing radial direction, it has been obtainedan increase in the radial component of the gas flow, with theconsequence that it is possible to further augment the flow rate of thegas phase and thus improve the mass transfer between the phases and withthem, the capacity of the column used to implement this process.

[0040] Alternatively, the same result can be obtained by causing theliquid phase and gas phase to flow in a substantially axial,respectively prevailing radial direction through a plurality ofsuperimposed filler-containing baskets.

[0041] Advantageously, according to the last-mentioned embodiment, theprocess according to the invention further comprises the step of:

[0042] collecting and re-distributing said liquid phase betweensuccessive baskets.

[0043] In this way, it is possible to maintain the whole fillerconstantly swept by the liquid phase, thus avoiding the latter fromflowing down along preferential paths which swept only local zones ofthe filler, resulting in a much reduced effective surface area beingavailable for the mass transfer.

[0044] Preferably, the gas phase is caused to flow radially throughcontiguous zones of said at least one basket or through successivebaskets, in opposite directions, thereby to provide a zigzag flowpathfor the gas phase within the column, which proves advantageous from theconstructional standpoint.

[0045] For implementing the above process, the invention advantageouslyprovides a filled-type column for effecting mass transfer between aliquid phase and a gaseous phase, which comprises:

[0046] an external shell;

[0047] at least one basket for containing the filler extending insidesaid shell, said at least one basket being through-penetrated by saidphases in countercurrent relationship;

[0048] respective means for feeding said liquid phase and said gaseousphase into said column;

[0049] respective means for extracting said liquid phase and saidgaseous phase from said column;

[0050] the column being characterized in that said at least one basketis provided with a gas-permeable surface, for passing the gaseous phase,which is larger than its cross-section.

[0051] Preferably, the column according to the invention ischaracterized in that said at least one basket has a smallercross-section than the shell cross-section and has opposite sidewallswhich are gas-permeable, and in that it further comprises means forcausing said gaseous phase to flow through said at least one basket in aprevailing radial flow.

[0052] According to a preferred embodiment, the column advantageouslycomprises:

[0053] an external shell, substantially cylindrical in shape;

[0054] at least one annular basket for containing the filler which isdisposed coaxially within said shell and includes opposite respectivelyinner and outer gas-permeable cylindrical walls, said at least onebasket being through-penetrated by said liquid phase in a substantiallyaxial flow direction;

[0055] a first free space defined between an inner wall of the shell andsaid outer wall of the basket;

[0056] a second free space defined inwardly of said inner wall of thebasket;

[0057] respective means for feeding said liquid phase and said gaseousphase into said column;

[0058] means for causing at least a major portion of said gaseous phaseto flow through said at least one basket, from said first free space tosaid second free space, or the other way round;

[0059] respective means for extracting said liquid phase and saidgaseous phase from said column.

[0060] According to a further preferred embodiment, the columnadvantageously comprises:

[0061] an external shell;

[0062] at least one basket for containing the filler, disposed insidesaid shell coaxially therewith and provided with opposite sidewalls,preferably plane, which are gas-permeable, said at least one basketbeing through-penetrated by said liquid phase in a substantially axialflow direction;

[0063] first and second free spaces, located opposite to each other anddefined between an inner wall of said shell and said sidewalls of thebasket;

[0064] respective means for feeding said liquid phase and said gaseousphase into said column;

[0065] means for causing at least a major portion of said gaseous phaseto flow through said at least one basket, from said first free space tosaid second free space or the other way round;

[0066] respective means for extracting said liquid phase and saidgaseous phase from said column.

[0067] According to a further aspect, the invention provides a method ofretrofitting a column for effecting mass transfer between a liquid phaseand a gaseous phase, being of either the filled or the plates type,which method is characterized in that it comprises the step of:

[0068] providing, inside said column, at least one basket for containinga filler and having a gas-permeable surface, for passing the gaseousphase, which is larger than its cross-section.

[0069] Thanks to the above method of retrofitting an existing column, aprocess for effecting mass transfer between a liquid phase and a gaseousphase can be obtained which allows to provide a high rate of transferbetween the phases in a simple and effective manner, at low investmentand operating costs, and with low energy consumption.

[0070] Further features and advantages of this invention will becomeapparent from the following description of an embodiment of the processaccording to the invention, given by way of non-limitative example withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0071] In the drawings:

[0072]FIG. 1 is a longitudinal section view of a first embodiment of afilled-type column, as constructed either from an original design or bymodifying a traditional column, to implement the process according tothe invention;

[0073]FIG. 2 is a longitudinal section view of the column shown in FIG.1, taken along line II-II in FIG. 1;

[0074]FIG. 3 is a longitudinal section view of a second embodiment of afilled-type column, as constructed either from an original design or bymodifying a traditional column, to implement the process according tothe invention;

[0075]FIG. 4 is a longitudinal section view of the column shown in FIG.3, taken along line IV-IV in FIG. 3;

[0076]FIG. 5 is a longitudinal section view of a third embodiment of afilled-type column, as constructed either from an original design or bymodifying a traditional column, to implement the process according tothe invention;

[0077]FIG. 6 is a longitudinal section view of the column shown in FIG.5, taken along line VI-VI in FIG. 5.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0078] Referring to FIGS. 1-6, generally shown at 1 is a filled-typecolumn for effecting mass transfer between a liquid phase and a gaseousphase, according to the invention.

[0079] The column 1 comprises an external shell 2, at least one basket 3extended inside the shell 2 to receive a filler 4, respective means 5and 6 for feeding a liquid phase and a gaseous phase into the column 1,and respective means 7 and 8 for extracting the liquid phase and gaseousphase from the column 1.

[0080] Means 5 and 7 for feeding, respectively extracting, the liquidphase to/from the column 1 generally comprise conduits, nozzle-typedispensers or collecting chambers of known types to the person skilledin the art, and accordingly, no further discussed in detail herein.

[0081] Similarly, means 6 and 8 for feeding, respectively extracting,the gaseous phase to/from the column 1 generally comprise gas inlets andoutlets of known types to the person skilled in the art, andaccordingly, no further discussed in detail herein.

[0082] In the examples of FIGS. 1, 3 and 5, the liquid phase is fed bythe means 5 to a top end of the column 1, and is extracted by the means7 from a bottom end thereof. Thus, the liquid phase will flow throughthe column 1 in a manner known per se, i.e. in a substantially axialdirection.

[0083] The gaseous phase is fed by the means 6 to a bottom end of thecolumn 1 and extracted from a top end thereof by the means 8.

[0084] The liquid phase and gaseous phase could, however, be fed in andextracted at intermediate locations on the column 1, as is usually donein traditional filled-type columns.

[0085] In other words, means for feeding, respectively extracting, theliquid phase and/or the gaseous phase may be provided anywhere along theshell 2, in order to maintain constant operational conditions within thecolumn or to improve the mass transfer between the phases.

[0086] According to the present invention, said at least one basket 3 isadvantageously provided with a gas-permeable surface for the passage ofthe gas phase through the filler 4, which is larger than itscross-section.

[0087] For the purpose, the basket 3 is given a smaller cross-sectionthan the cross-section of the shell 2, and has opposite sidewalls 9 and10 which are gas-permeable.

[0088] In addition, the column 1 advantageously includes appropriatemeans, to be described, adapted to cause the gas phase to flow throughthe basket 3 in a prevailing radial direction.

[0089] In this way, the gas phase is caused to flow through the column1—contrary to the constant teaching of the prior art—in countercurrentto the liquid phase along a prevailing radial flow direction.

[0090] The gas-permeable surface of the sidewalls 9 and 10 isadvantageously provided larger than the cross-section of the basket 3.

[0091] According to the embodiment shown in FIG. 1, the filled-typecolumn 1 of this invention comprises an external shell 2 which issubstantially cylindrical in shape, and at least one annular basket 3a-3 e for containing the filler 4 which is mounted inside the shell 2coaxially therewith and has opposite gas-permeable cylindrical inner andouter walls, 9 and 10 respectively.

[0092] A first free space 11 is formed between an inner wall 2 a of theshell and the outer wall 10 of the basket, and a second free space 12 isformed within the inner wall 9 of the basket.

[0093] Advantageously, the column 1 further comprises means 9 a, 10 a,14, 15, for causing at least a major portion of the gas phase to flowthrough said at least one basket 3 a-3 e, from the first free space 11to the second free space 12 or the other way round.

[0094] In the example of FIG. 1, the column 1 contains five annularbaskets 3 a-3 e for containing the filler 4, which are superimposed anddisposed coaxially within the shell 2.

[0095] These baskets are flown by the liquid phase from above downwardof the column 1 with substantially axial direction, and by the gas phasefrom beneath upward with substantially axial-radial direction, asindicated by arrows Fg.

[0096] The number of baskets 3 inside the shell 2 may vary to suit thesize of the column 1 and the rate of mass transfer sought. A number ofbaskets in the one to twenty range should prove adequate to meet mostrequirements.

[0097] To promote the mass transfer between the phases and ensureoptimum operating conditions throughout the column 1, the baskets 3 a-3e for containing the filler 4 may advantageously be provided indifferent lengths, e.g. with the lowermost basket 3 a being the longestand the other baskets decreasing gradually in length to the uppermostbasket 3 e which would be the shortest.

[0098] The baskets 3 a-3 e shown in FIG. 1 are laid contiguously, andhave each a perforated bottom 13 which can be permeated by both theliquid phase and the gaseous phase.

[0099] Advantageously, the perforated bottoms 13 of the baskets 3 a-3 eare suitably sized to deflect the gas phase radially and/or to collectand re-distribute the liquid phase between the baskets.

[0100] The presence of the perforated bottoms 13 between successivecontiguous baskets 3 a-3 e produces a controlled increase ofpredetermined entity in pressure drop of the gas phase flowing throughthe basket which allows at least some of that phase to be deflectedradially, and the outflowing liquid phase from one basket to beoptionally collected and re-distributed to the next basket.

[0101] By having the liquid phase collected and re-distributed betweensuccessive baskets 3 a-3 e, optimum conditions can be ensured for themass transfer between the phases, with the entire surface area of thefiller 4 being utilized and preventing the formation of preferentialflowpaths for the liquid phase going through the filler.

[0102] The portion of the gas phase which has been deflected radiallyout of one basket 3 a-3 d is then re-introduced into the next basket 3b-3 e, again in radial flow direction.

[0103] Preferably, the radial flow of the gas phase through the baskets3 a-3 e is either of centrifugal or centripetal type.

[0104] Particularly satisfactory results have been obtained using analternating centripetal and centrifugal flow (or vice versa), asindicated by the arrows Fg in FIG. 1. That is, the gaseous phase goesradially through successive baskets 3 a-3 e in opposite directions.

[0105] The baskets 3 a-3 e, or just some of them, may be disposed insidethe column 1 at suitable distances from one another to define respectivecylindrical spaces between successive baskets.

[0106] These spaces may advantageously accommodate means (not shown) ofcollecting and re-distributing the liquid phase from one basket to thenext, which means are conventional and no further described herein.

[0107] Alternatively to having a plurality of baskets, the column 1according to the present invention could advantageously have a singlebasket 3 divided into a plurality of zones (indicated in FIG. 1 by thesame references 3 a-3 e as the baskets), contiguous one above the other.This arrangement is to be regarded as substantially equivalent to thebasket arrangement shown in FIG. 1.

[0108] According to this embodiment, no perforated bottoms 13 areprovided, and the filler 4 inside the basket 3 forms a continuous singlebody.

[0109] According to a particularly advantageous aspect of the invention,the outer wall 10, or the inner wall 9, of at least one basket 3 a-3 ecomprises a portion 10 a, respectively 9 a, which is gas-impermeable andof a predetermined length along a top end thereof.

[0110] In the example of FIG. 1, both the outer wall 10 and the innerwall 9 of each basket 3 a-3 c advantageously comprise thegas-impermeable portion 10 a and 9 a, respectively.

[0111] Where, instead of the baskets 3 a-3 c, the column 1 comprises asingle basket 3 divided into a plurality of zones 3 a-3 c as describedabove, the gas-impermeable portions 9 a and 10 a of predetermined lengthof the inner wall 9 and/or of the outer wall 10, respectively, of thebasket 3 would be defined along a top end of each zone.

[0112] By providing a gas-impermeable top portion on at least one ofthe, preferably both, sidewalls of the baskets or zones 3 a-3 c, a majorportion of the gas phase stream flowing from bottom to top through thefiller 4 can be deflected radially in a simple and effective manner,allowing the gas phase to flow through the column 1 in a prevailingradial direction.

[0113] In fact, the unperforated portion of the sidewalls 9 and/or 10produces a controlled increase (of predetermined entity) in the pressuredrop of the gas phase which advantageously impedes the axial flowthrough the filler 4, thereby deflecting the gas phase stream radially.

[0114] The proportion of the gas phase that goes radially, respectivelyaxially, through the filler 4 is dictated by the lengths or extents ofthe gas-impermeable portions 9 a and/or 10 a, because the pressure dropundergone by the gas phase would vary correspondingly with suchportions. By increasing the length of the gas-impermeable portion, theresistance to the axial flow and, hence, the radial flow component ofthe gas phase is increased.

[0115] As shown in FIG. 1, it is only a minor portion of the gas phasethat goes through the filler 4 in a substantially axial flow, while themajor portion thereof follows substantially radial flowpaths, resultingin a mixed flow being established, i.e. axial-radial.

[0116] Where both sidewalls 9 and 10 of the basket(s) 3 a-3 e includegas-impermeable portions 9 a and 9 b, the length of each portion mayadvantageously be different for the inner wall 9 and the outer wall 10,so as to augment or diminish the radial component of the gas phase flow.

[0117] Particularly satisfactory results have been obtained with thegas-impermeable portion 9 a, 10 a being 5% to 30% of the length of theinner wall 9 and/or outer wall 10 of the basket 3 a-3 e.

[0118] Preferably, the length of the gas-impermeable portion will be inthe range of 10% to 25%.

[0119] According to a further particularly advantageous aspect of thisinvention, the column 1 includes gas-impermeable means 14 and 15 forclosing the free spaces 11 and 12 arranged at the first and at thesecond free space 11 and 12 respectively.

[0120] According to the example of FIG. 1, in at least one basket 3 a-3e, these closure means advantageously comprise an annular baffle 14which is gas-impermeable and disposed at a top end of the first freespace 11, and a circular baffle 15 which is gas-impermeable and disposedat a bottom end of the second free space 12.

[0121] In this way, the gaseous phase can be caused to pass from onebasket 3 a-3 d to the next basket 3 b-3 e in an alternating centripetaland centrifugal flow of prevailing radial direction, while preventingthe gas phase from bypassing one or more of the baskets, which wouldadversely affect the overall efficiency of the mass transfer takingplace between the phases.

[0122] In fact, when at least a major portion of the gas phase is causedto flow radially through the filler 4, it matters that the gas exiting agiven basket be directed to the next basket in such a manner that itcannot flow out and sweep across other baskets in the column or evenescape from the column.

[0123] Where the gaseous phase is to move from one basket 3 a-3 d to thenext 3 b-3 e in a mainly radial flow of the alternating centrifugal andcentripetal type, the gas-impermeable annular baffle 14 would bedisposed at a bottom end of the first free space 11, and thegas-impermeable circular baffle 15 would be disposed at a top end of thesecond free space 12.

[0124] In the example of FIG. 1, with the column 1 being traversed by asubstantially axial-radial flow of the gaseous phase, the baskets 3 a-3e are left open at their top ends, rather than being closed by agas-impermeable cover.

[0125] This greatly facilitates the axial traversing of the same by theliquid phase, and particularly in the upper basket 3 e, facilitates thefeeding and distributing of this phase on the filler 4 thereincontained.

[0126] In FIG. 1, the numeral 16 generally indicates a heat exchangerdisposed in the second free space 12 in the basket 3 c, for subjectingthe gaseous phase which leaves the basket 3 c with radial flow and isdirected to the next basket 3 d, to an indirect heat exchange with aheating or cooling fluid, such as water.

[0127] In particular, the gaseous phase is caused to flow through theheat exchanger 16 on the shell side, with the heating or cooling fluidon the tube side. For simplicity, the means for feeding and extractingthe heating or cooling fluid to/from the heat exchanger 16 have beenomitted from FIG. 1 because known per se.

[0128] Depending on individual requirements, one or more heat exchangers16 providing indirect heat exchange for at least some of the gaseousphase flowing between successive baskets 3 a-3 e may be arranged in thefree space 12.

[0129] By providing a heat exchanger 16 in the column 1, the scrubbing,absorbing distilling and decomposing processes for which the column isintended can all be improved.

[0130] As mentioned above, all the features, except the gas permeablebottom 13, described in relation to the baskets 3 a-3 e also apply tothe particular embodiment, not shown, of this invention wherein thecolumn 1 comprises a single basket 3 divided into a plurality of zones 3a-3 e.

[0131]FIG. 3 shows an embodiment of a column 1 for implementing theprocess according to the present invention, which differs from theexample of FIG. 1 mainly in that the gaseous phase is caused to flowthrough the filler 4 in a substantially radial direction, rather thanaxial-radial.

[0132] In this figure, the parts of the column 1 which are structurallyand functionally equivalent to those shown in FIG. 1, carry the samereference numbers and will be not further described.

[0133] Advantageously, the gaseous phase is caused to flow insubstantially radial direction by closing the top end of the basket(s) 3loaded with the filler 4 disposed inside the column 1.

[0134] For the purpose, at least one basket 3 has a gas-impermeablecover 17 mounted at its top end.

[0135] In the example of FIG. 3, the baskets 3 a and 3 b are eachprovided with a cover 17 such that not even a minor portion of thegaseous phase will be allowed to go through the filler 4 in an axialflow direction.

[0136] Thanks to the gas-impermeable cover 17, the gaseous phase isforced to flow radially through the filler 4, and is prevented fromexiting the baskets 3 a-3 b in an axial direction.

[0137] In this case, the gas-impermeable portion 9 a, respectively 10 a,of the inner and outer sidewalls 9 and 10 of the baskets 3 a-3 b,additionally to deflecting the gaseous phase flow in a radial direction,advantageously prevents the baskets 3 a-3 b from being undesirablybypassed by at least a portion of said phase which might leak, throughany gap between the cover 17 and the filler 4, directly from the firstfree space to the second, 11 respectively 12, or the other way round,affecting the overall efficiency of the mass transfer between thephases.

[0138] Preferably, the gaseous phase will flow, as indicated by thearrows Fg in FIG. 3, radially outwards through the first basket 3 a andradially inwards through the second basket 3 b.

[0139] The radial direction of the gaseous phase may be selected to suitthe column 1 construction, and may be purely centripetal or purelycentrifugal, or centripetal and centrifugal (or vice versa). Similar tothe example shown in FIG. 1, the advantages resulting by selecting aparticular direction for the gaseous phase flow do concern theconstruction of the column 1, but are of trivial importance to theefficiency of the mass transfer between the phases.

[0140] Furthermore, a combined process could be provided with some ofthe baskets 3 being traversed by the gaseous phase in substantiallyradial flow direction, and some others in axial-radial direction.

[0141] In this case, not all of the baskets 3 a-3 b would be fitted witha gas-impermeable cover 17.

[0142] The number of baskets 3 a-3 b shown in FIG. 3 and their differentlengths are merely illustrative, and may vary according to necessity aspreviously described in relation to FIG. 1.

[0143] It should be noted that the means 5 for feeding the liquid phaseinto the column 1 locates between the cover 17 of the upper basket 3 band the filler 4.

[0144] According to this embodiment, the liquid phase flowing out of theupper basket 3 b is picked up—by means not shown because known perse—and re-distributed to the lower basket 3 a, for example, by a nozzletype dispenser 18, itself disposed between a respective cover 17 and thefiller 4.

[0145] Advantageously, the baskets 3 a and 3 b locate adjacent to eachother, with the cover 17 of the lower basket 3 a forming thegas-impermeable bottom of the upper basket 3 b. A column of compact andsimple construction is thus obtained. However, a plurality of baskets 3spaced apart inside the column 1 could be provided instead.

[0146] Although not shown in FIG. 3, the free space 12 mayadvantageously accommodate one or more heat exchangers for an indirectheat exchange between the gaseous phase flowing from one basket to thenext and a heating or cooling fluid, as described in relation to FIG. 1.

[0147] According to a further embodiment of this invention depicted inFIG. 5, the column 1 includes at least one basket 3 a-3 c for containingthe filler 4 and being disposed coaxially within the shell 2 and havingopposite sidewalls 19, 20 which are gas-permeable and preferably plane.

[0148] Defined between the inner wall 2 a of the shell 2 and thesidewalls 19 and 20 of the baskets 3 a-3 c are first and second freespaces 21 and 22, opposite to each other.

[0149] Advantageously, appropriate means (19 a, 20 a, 23) are providedfor causing at least a major portion of the gaseous phase to flow intothe column 1 through at least one basket 3 a-3 c, from the first freespace 21 to the second free space, or the other way round.

[0150] In FIG. 5, the parts of the column 1 which are structurally andfunctionally equivalent to those shown in the previous figures areindicated by the same reference numbers and will be no furtherdescribed.

[0151] According to this embodiment of the invention, the liquid phaseis caused to flow through the filler 4 from above downward in asubstantially axial direction, whereas the gaseous phase advantageouslyflows through the filler 4 in a substantially crosswise direction frombottom to top.

[0152] In other words, the gaseous phase is moved from one side of thebasket 3 a-3 c to the other in an essentially oblique flow wherein thehorizontal component prevails.

[0153] Preferably, as shown in FIGS. 5 and 6, the shell 2 issubstantially cylindrical in shape, and the cross-sectional shape of thefree spaces 21 and 22 is a circular sector. In this way, the spaceavailable inside the column 1 can be best utilized, and roomy baskets 3a-3 c for containing the filler 4, as well as sufficiently broad freespaces 21 and 22 to allow the gaseous phase to flow through withoutundue frictional drag, can be provided.

[0154] Similarly as the previous examples, here again at least one ofthe sidewalls 19 and 20, preferably both, of the basket 3 a-3 ccomprises a gas-impermeable portion 19 a, respectively 20 a, ofpredetermined length at a top end thereof, which is effective to deflectthe gaseous phase flow horizontally.

[0155] In addition, the column 1 advantageously includes gas-impermeablemeans 23 for closing the free spaces 21 and 22, disposed at the firstand at the second free space 21 and 22.

[0156] For the purpose, the closure means comprise a gas-impermeablebaffle 23 located at a top end of the first free space 21, and agas-impermeable baffle 23 located at a bottom end of the second freespace 22.

[0157] In an alternative embodiment, not shown, the gas-impermeablebaffles 23 are located at a bottom end of the first free space 21 and ata top end of the second free space 22, respectively.

[0158] In the example of FIG. 5, these baffles 23 have a circular-sectorshape.

[0159] The baffles 23 are advantageously effective to induce a crossflowof the gaseous phase through the filler 4 from the first free space 21to the second 22 and/or the other way round, while preventing thegaseous phase from undesirably bypassing the baskets 3 a-3 c.

[0160] As concerns the length of the gas-impermeable portion 19 a and 20a of the sidewalls 19 and 20, as well as the number, the arrangement andthe length of the baskets 3 a-3 c inside the column 1, the samecharacteristics and advantages apply as have been discussed above inrelation to the previous figures.

[0161] Just as example, it is noted that in column 1 of FIG. 5 threecatalytic baskets 3 a-3 c are coaxially arranged, loaded with the filler4, superimposed and contiguous with one another, with the top basket 3 cbeing longer than the underlying baskets 3 a-3 b.

[0162] A perforated bottom 13 is provided in each basket 3 a-3 c forpassing the liquid phase and a minor portion of the gaseous phasebetween successive baskets. In addition, the baskets 3 a-3 c are openended at the top.

[0163] As a result, in the example of FIG. 5, the gaseous phase isadvantageously allowed to flow through the baskets 3 a-3 c in a mixedcrosswise and axial direction.

[0164] Alternatively, in an embodiment not shown in the drawings, thebaskets 3 a and 3 c may be fitted with respective gas-impermeable coversat their top ends, so that a true crosswise flow of the gaseous phasecan be provided through the filler 4.

[0165] Also within the scope of this invention is a column 1 comprisingsome of the baskets flown by the gaseous phase in a substantiallycrosswise direction and others of the baskets flown in a mixed crosswiseand axial direction.

[0166] The baskets 3 a-3 c may advantageously be disposed in a mutuallyspaced-apart relationship inside the column 1. Also, suitable means maybe provided between successive baskets 3 a-3 c to collect andre-distribute the liquid phase.

[0167] In the example of FIG. 5, these means comprise the perforatedbottoms 13 provided at the bottom ends of the baskets 3 a-3 c, which issuitably sized to deflect the gaseous phase flow crosswise and/or tocollect and redistribute the liquid phase flowing between successivebaskets.

[0168] Alternatively, the column 1 could advantageously comprise,instead of the baskets 3 a-3 c, a single basket 3 divided into aplurality of contiguously superimposed zones, not shown, which can belikened in every respect to the above-described baskets 3 a-3 c and havethe same characteristics and advantages. Of course, no perforatedbottoms 13 or gas-impermeable covers would be provided between suchcontiguous zones.

[0169] A column 1 could also be arranged to incorporate a plurality ofbaskets 3 a-3 c, with at least one of the baskets being divided into theaforesaid plurality of zones.

[0170] According to an embodiment, not shown but particularlyadvantageous, of this invention, a substantially radial or crosswiseflow of the gaseous phase can be provided through a column 1 of the typedescribed in relation to FIG. 3 or 5, by replacing the gas-impermeablecovers with gas-permeable perforated covers, and advantageouslyexploiting the hydrostatic liquid head that forms at such perforatedcovers to stop the gaseous phase from flowing axially through thebasket.

[0171] In other words, the liquid phase—which is caused to flow fromabove downward through the column 1—upon leaving one basket, collects atthe perforated cover of the next basket, and before being re-distributedand going through this next basket, forms a horizontal liquid layer thatcannot be penetrated by the gaseous phase, so that the gaseous phasewill be obliged to flow in a crosswise or radial direction.

[0172] The present invention is not limited by a specific type of filler4, and any filler types may be used inside the column 1, includingtexturized or random types. Advantageously, fillers 4 of different typemay be interposed between baskets or between zones of one basket, sothat the gaseous phase flow rate, and the mass transfer between thephases flowing through the column 1, can be controlled or modified.

[0173] The filler 4 may be made of inert material with respect to theflowing liquid and gaseous phases or it can be a suitable catalyst, ormixtures thereof. In case a catalyst is used in the column 1, thenchemical reactions may take place in addition to the mass transferbetween the phases.

[0174] As brought out by the embodiments described in relation to FIGS.1-6, according to the process of the present invention for effectingmass transfer between a liquid phase and a gaseous phase inside afilled-type column 1 comprising an external shell 2 which accommodatesat least one basket 3 for containing the filler 4 wherethrough suchphases are caused to flow in countercurrent, the gaseous phase isadvantageously fed into said at least one basket 3 through agas-permeable surface (9, 10, 19, 20) thereof which is larger than thebasket cross-section.

[0175] Preferably, this process for effecting mass transfer between aliquid phase and a gaseous phase is carried out within a filled-typecolumn 1 comprising an external shell 2 which accommodates at least onebasket 3 for containing the filler 4 and having a smaller cross-sectionthan the cross-section of the shell 2, and comprises the steps offeeding (means 5 and 6) the liquid and gaseous phases into thefilled-type column 1, causing the liquid phase to flow through said atleast one basket 3 in a substantially axial direction, causing thegaseous phase to flow through said at least one basket 3 in a prevailingradial direction (means 9 a, 10 a, 14, 14, 19 a, 20 a, 23), andextracting (means 7 and 8) the liquid and gaseous phases from thefilled-type column 1.

[0176] As previously described, a reduction in the pressure drop of thegaseous phase flowing through the filler 4 can be provided by thepresent process, whereby optimum flow rates and velocities can beachieved for that phase (and for the liquid phase) effective to augmentthe mass transfer between the liquid phase and the gaseous phase.

[0177] Additionally thereto, it has been found unexpectedly that bycausing the gaseous phase to flow mainly radially through the filler 4,superior efficiency can be achieved for the mass transfer since thedriving force which is responsible for such transfer is positively actedupon.

[0178] The operating conditions (such as pressure and temperature) ofthe present process may be changed within broad limits to suit the typesof fluids to be processed and any chemical operations to be carried out.

[0179] The present invention can also be advantageously applied incolumns having both plates and filler containing baskets

[0180] The column shown in FIGS. 1, 3 and 5 may be a brand new one, oralternatively be—according to an advantageous and preferred aspect ofthis invention—a pre-existing column of the filled or plates typeretrofitted for carrying out mass transfer between a liquid phase and agaseous phase.

[0181] Advantageously, the method for retrofitting such a column wouldcomprise the step of providing within the same at least a basket 3 forcontaining a filler 4 which has a gas-permeable surface, for passing thegaseous phase, which is larger than its cross-section.

[0182] In particular, the retrofitting method according to thisinvention is characterized in that it comprises the steps of providing,inside the pre-existing column, at least one basket 3 for containing thefiller 4 which has a smaller cross-section than the column cross-sectionand has opposite gas-permeable sidewalls 9, 10, 19, 20, and ofproviding, inside the column, means 9 a, 10 a, 14, 15, 19 a, 20 a, 23for causing the gaseous phase to flow through said at least one basketin a prevailing radial direction.

[0183] To obtain a column 1 of the same type as described in relation toFIG. 1 or 3, this retrofitting method advantageously comprises the stepsof providing at least one annular basket 3 for containing the filler 4disposed coaxially within the column 1, which basket 3 has oppositecylindrical gas-permeable inner and outer walls, 9 and 10, defining afirst free space 11 between an inner wall 2 a of the column 1 and theouter wall 10 of the basket 3, and defining a second free space 12formed inwardly of the inner wall 9 of the basket 3, and providing means9 a, 10, 14 and 15 for causing at least a major portion of the gaseousphase to flow through said at least one basket 3, from the first freespace 11 to the second free space 12 or the other way round.

[0184] To obtain a column 1 of the type described in relation to FIG. 5,the method advantageously comprises the steps of providing at least onebasket 3 for containing the filler 4 disposed coaxially within thecolumn 1, which basket 3 has opposite, preferably plane, gas-permeablesidewalls 19 and 20 defining a first and a second free space 21 and 22located opposite to each other between an inner wall 2 a of the column 1and the sidewalls 19-20 of the basket 3, and providing means 19 a, 20 a,23 for causing said at least one major portion of the gaseous phase toflow through said at least one basket from the first free space 21 tothe second free space 22 or the other way round.

[0185] Thanks to the present method for retrofitting pre-existingcolumns for effecting mass transfer between a liquid phase and a gaseousphase, it is advantageously possible to achieve a significant increasein the column capacity, and a reduction in energy consumption, becausethe column once modernized can operate more effectively at much largerflow rates of the liquid and gaseous phases.

[0186] Moreover, in accordance with further embodiments of theretrofitting method according to the present invention, additional meansor features can be advantageously provided, as defined in dependentclaims 32-37 and 39-44 appended hereto.

[0187] Further structural and processing advantages accruing from thepresent retrofitting method can be inferred from the previousdescription of FIGS. 1-6.

[0188] It can be appreciated that, in implementing the retrofittingmethod according to the invention, at least some of the constructionalelements originally included in the column would have to be removed in apreliminary step, it being possible to re-use such elements, forexample, to provide bottoms or covers for the new filler-containingbaskets.

[0189] From the foregoing description emerge clearly the numerousadvantages achieved by the present invention; in particular, it isobtained a process for effecting highly efficient mass transfer betweena liquid phase and a gaseous phase, in a simple and reliable manner, atlow investment and operating costs, and with low energy consumption.

1. Process for effecting mass transfer between a liquid phase and agaseous phase within a filled-type column which comprises an externalshell accommodating at least one filler-containing basket wherethroughsaid phases are caused to flow in a countercurrent relationship,characterized in that it comprises the step of feeding said gaseousphase to said at least one basket through a gas-permeable basket surfacewhich is larger than the basket cross-section.
 2. Process for effectingmass transfer between a liquid phase and a gaseous phase within afilled-type column which comprises an external shell accommodating atleast one filler-containing basket whose cross-section is smaller thanthe cross-section of said shell, said process comprising the steps of:feeding said liquid phase and gaseous phase into said filled-typecolumn; causing the liquid phase to flow through said at least onebasket in a substantially axial direction; causing the gaseous phase toflow through said at least one basket in a prevailing radial direction;extracting said liquid phase and gaseous phase from said filled-typecolumn.
 3. Process according to claim 2, characterized in that saidgaseous phase is caused to flow through said at least one basket in asubstantially radial, axial-radial, or crosswise direction.
 4. Processaccording to claim 2, characterized in that said liquid phase and saidgaseous phase are caused to flow in said at least one basket through aplurality of contiguously superimposed zones.
 5. Process according toclaim 2, characterized in that said liquid phase and said gaseous phaseare caused to flow in a direction substantially axial and prevailingradial respectively, through a plurality of superimposedfiller-containing baskets.
 6. Process according to claim 5,characterized in that it further comprises the step of: collecting andre-distributing said liquid phase between successive baskets.
 7. Processaccording to claims 4 and 5, characterized in that it further comprisesthe step of: subjecting at least a portion of said gaseous phase to anindirect heat exchange while said gaseous phase is being flowed from onezone to the next, respectively from one basket to the next. 8.Filled-type column for mass transfer between a liquid phase and agaseous phase, comprising: an external shell (2); at least one basket(3) for containing the filler (4) extending inside said shell (2), saidat least one basket (3) being through-penetrated by said phases incountercurrent relationship; respective means (5,6) for feeding saidliquid phase and said gaseous phase into said column; respective means(7,8) for extracting said liquid phase and said gaseous phase from saidcolumn; characterized in that said at least one basket (3) is providedwith a gas-permeable surface, for passing the gaseous phase, which islarger than its cross-section.
 9. Column according to claim 8,characterized in that said at least one basket (3) has a smallercross-section than the cross-section of the shell (2) and has oppositesidewalls (9, 10, 19, 20) which are gas-permeable, and in that itfurther comprises means (9 a, 10 a, 14-15, 19 a, 20 a, 23) for causingsaid gaseous phase to flow through said at least one basket (3) in amainly radial flow.
 10. Column according to claim 8, characterized inthat it comprises: an external shell (2), substantially cylindrical inshape; at least one annular basket (3 a-3 e) for containing the filler(4) which is disposed coaxially within said shell (2) and includesopposite respectively inner and outer gas-permeable cylindrical walls(9,10), said at least one basket (3 a-3 e) being through-penetrated bysaid liquid phase in a substantially axial flow direction; a first freespace (11) defined between an inner wall (2 a) of the shell (2) and saidouter wall (10) of the basket (3 a-3 e); a second free space (12)defined inwardly of said inner wall (9) of the basket (3 a-3 e);respective means (5,6) for feeding said liquid phase and said gaseousphase into said column; means (9 a, 10 a, 14, 15) for causing at least amajor portion of said gaseous phase to flow through said at least onebasket (3 a-3 e), from said first free space (11) to said second freespace (12), or the other way round; respective means (7,8) forextracting said liquid phase and said gaseous phase from said column.11. Column according to claim 10, characterized in that said at leastone basket (3) is divided into a plurality of contiguously superimposedzones (3 a-3 e), said means (9 a, 10 a, 14, 15) for causing at least amajor portion of said gaseous phase to flow through the basket (3) fromsaid first free space (11) to said second free space (12), or the otherway round, being arranged at each of said zones (3 a-3 e).
 12. Columnaccording to claims 10 and 11, characterized in that said basket outerwall (10) and/or said basket inner wall (9) comprise, at a top end ofsaid at least one basket (3 a-3 e), respectively at a top end of eachzone (3 a-3 e), a gas-impermeable portion (9 a,10 a) of predeterminedlength, the length of said gas-impermeable portion (9 a,10 a) beingpreferably a different length for the basket outer (10) and inner (9)walls.
 13. Column according to claim 12, characterized in that saidgas-impermeable portion (9 a, 10 a) stretches over 5% to 30% of thelength of said outer (10) and/or inner (9) walls of the basket (3 a-3e), respectively of the length of said zone (3 a-3 e).
 14. Columnaccording to any one of claims 10 to 13, characterized in that said atleast one basket (3 a-3 e), respectively each of said zones (3 a-3 e),comprises gas-impermeable closure means (14,15) for said free spaces(11,12) disposed at said first and said second free space (11,12). 15.Column according to claim 14, characterized in that said closure means(14,15) comprise an annular gas-impermeable baffle (14) disposed at abottom end of said first free space (11), respectively of said zones (3a-3 e), and a circular gas-impermeable baffle (15) disposed at a top endof said second free space (12), respectively of said zones (3 a-3 e), oran annular gas-impermeable baffle (14) disposed at a top end of saidfirst free space (11), respectively of said zones (3 a-3 e), and acircular gas-impermeable baffle (15) disposed at a bottom end of saidsecond free space (12), respectively of said zones (3 a-3 e).
 16. Columnaccording to claim 10, characterized in that it comprises a plurality ofannular baskets (3 a-3 e) for containing the filler (4) which aresuperimposed and disposed coaxially within said shell (2), said baskets(3 a-3 e) being preferably contiguous with one another and/or differentin length.
 17. Column according to claims 11 and 16, characterized inthat it comprises at least one heat exchanger (16) arranged in saidsecond free space (12) to provide an indirect heat exchange with atleast some of the gaseous phase flowing from one zone (3 a-3 d) to thenext (3 b-3 e) of said at least one basket (3), respectively from onebasket (3 a-3 d) to the next (3 b-3 e).
 18. Column according to claim 8,characterized in that it comprises: an external shell (2), preferablysubstantially cylindrical in shape; at least one basket (3 a-3 c) forcontaining the filler (4), disposed inside said shell (2) coaxiallytherewith and provided with opposite sidewalls (19,20), preferablyplane, which are gas-permeable, said at least one basket (3 a-3 c) beingthrough-penetrated by said liquid phase in a substantially axial flowdirection; first and second free spaces (21,22), located opposite toeach other and defined between an inner wall (2 a) of said shell (2) andsaid sidewalls (19,20) of the basket (3 a-3 c), said free spaces havinga preferably circular-sector shape in cross-section; respective means(5,6) for feeding said liquid phase and said gaseous phase into saidcolumn; means (19 a, 20 a, 23) for causing at least a major portion ofsaid gaseous phase to flow through said at least one basket (3 a-3 c),from said first free space (21) to said second free space (22) or theother way round; respective means (7,8) for extracting said liquid phaseand said gaseous phase from said column.
 19. Column according to claims10 and 18, characterized in that said at least one basket (3 a-3 e) isopen-ended at the top.
 20. Column according to claims 10 and 18,characterized in that said at least one basket (3 a-3 e) comprises agas-impermeable cover (17) at its top end.
 21. Column according to claim18, characterized in that said at least one basket (3) is divided into aplurality of contiguously superimposed zones (3 a-3 c), said means (19a, 20 a, 23) for causing at least a major portion of said gaseous phaseto flow through the basket (3) from said first free space (21) to saidsecond free space (22), or the other way round, being provided at eachof said zones (3 a-3 c).
 22. Column according to claims 18 and 21,characterized in that at least one of said basket sidewalls (19,20)comprises, at a top end of said at least one basket (3 a-3 c),respectively at a top end of each zone (3 a-3 c), a gas-impermeableportion (19 a,20 a) of predetermined length, the length of saidgas-impermeable portion (19 a,20 a) being preferably a different lengthfor either basket sidewalls (19,20).
 23. Column according to claim 22,characterized in that said gas-impermeable portion (19 a,20 a) stretchesover 5% to 30% of the length of said sidewalls of the basket (3 a-3 c),respectively of the length of said zone (3 a-3 c).
 24. Column accordingto any one of claims 18 to 23, characterized in that said at least onebasket (3 a-3 c), respectively each of said zones (3 a-3 c), comprisesgas-impermeable closure means (23) for said free spaces disposed at saidfirst and said second free space (21,22).
 25. Column according to claim24, characterized in that said closure means comprise a gas-impermeablebaffle (23) disposed at a bottom end of said first free space (21),respectively of said zones (3 a-3 c), and a gas-impermeable baffle (23)disposed at a top end of said second free space (22), respectively ofsaid zones (3 a-3 c), or a gas-impermeable baffle (23) disposed at a topend of said first free space (21), respectively of said zones (3 a-3 c),and a gas-impermeable baffle (23) disposed at a bottom end of saidsecond free space (22), respectively of said zones (3 a-3 c).
 26. Columnaccording to claim 18, characterized in that it comprises a plurality ofbaskets (3 a-3 c) for containing the filler (4) which are superimposedand disposed coaxially within said shell (2), said baskets (3 a-3 c)being preferably contiguous with one another and/or different in length.27. Column according to claims 16 and 26, characterized in that saidbaskets (3 a-3 c) have a perforated bottom (13).
 28. Column according toclaim 27, characterized in that said perforated bottom (13) is suitablysized to deflect the gaseous phase in radial direction, respectivelycrosswise direction, and/or to collect and re-distribute the liquidphase between successive baskets (3 a-3 c).
 29. Method of retrofitting acolumn for effecting mass transfer between a liquid phase and a gaseousphase, being of either the filled or the plates type, which method ischaracterized in that it comprises the step of: providing, inside saidcolumn, at least one basket (3) for containing a filler (4) and having agas-permeable surface, for passing the gaseous phase, which is largerthan its cross-section.
 30. Method according to claim 29, characterizedin that it comprises the steps of: providing, inside said column, atleast one basket (3) for containing the filler (4) which has a smallercross-section than the column cross-section and has oppositegas-permeable sidewalls (9, 10, 19, 20); providing, inside said column,means (9 a, 10 a, 14, 15, 19 a, 20 a, 23) for causing said gaseous phaseto flow through said at least one basket (3) in a prevailing radialdirection.
 31. Method according to claim 29, characterized in that itcomprises the steps of: providing at least one annular basket (3 a-3 e)for containing the filler (4) disposed coaxially within said column (2),said basket having opposite cylindrical gas-permeable inner and outerwalls (9,10) defining a first free space (11) between an inner wall (2a) of the column and said outer wall (10) of the basket (3 a-3 e), anddefining a second free space (12) formed inwardly of said inner wall (9)of the basket (3 a-3 c); providing means (9 a, 10 a, 14-15) for causingat least a major portion of said gas phase to flow through said at leastone basket (3 a-3 e), from said first free space (11) to said secondfree space (12), or the other way round.
 32. Method according to claim31, characterized in that said at least one basket (3) is divided into aplurality of contiguously superimposed zones (3 a-3 e), said means (9 a,10 a, 14-15) for causing at least a major portion of said gaseous phaseto flow through the basket from said first free space (11) to saidsecond free space (12) being provided at each of said zones (3 a-3 e).33. Method according to claims 31 and 32, characterized in that saidbasket outer wall (10) and/or said basket inner wall (9) comprises, at atop end of said at least one basket (3 a-3 e), respectively at a top endof each zone (3 a-3 e), a gas-impermeable portion (9 a,10 a) ofpredetermined length, the length of said gas-impermeable portion (9 a,10 a) being preferably a different length for the basket outer (10) andinner (9) walls.
 34. Method according to any one of claims 31 to 33,characterized in that it further comprises the step of: providinggas-impermeable closure means (14,15) for said free spaces (11,12)disposed at said first and said second free space (11,12) of said atleast one basket (3 a-3 e) respectively of each of said zones (3 a-3 e).35. Method according to claim 34, characterized in that it comprises thesteps of: providing a gas-impermeable annular baffle (14) at either abottom or a top end of said first free space (11), respectively of saidzones (3 a-3 e); providing a gas-impermeable circular baffle (15) ateither a top or a bottom end of said second free space (12),respectively of said zones (3 a-3 e).
 36. Method according to claim 31,characterized in that it comprises the step of: providing a plurality ofannular baskets (3 a-3 e) for containing the filler (4), superimposedand disposed coaxially within said column.
 37. Method according toeither claim 32 or 36, characterized in that it further comprises thestep of: providing at least one heat exchanger (16) in said second freespace (12) for indirectly exchanging heat with at least some of thegaseous phase flowing from one zone (3 a-3 d) to the next (3 b-3 e) ofsaid at least one basket, or from one basket (3 a-3 d) to the next (3b-3 e).
 38. Method according to claim 29, characterized in that itcomprises the steps of: providing at least one basket (3 a-3 c) forcontaining the filler (4) disposed coaxially within said column andprovided with opposite, preferably plane, gas-permeable sidewalls(19,20) defining first and second free spaces (21,22) located oppositeto each other between an inner wall (2 a) of said column and saidsidewalls (19,20) of the basket; providing means (19 a, 20 a, 23) forcausing at least a major portion of said gaseous phase to flow throughsaid at least one basket (3 a-3 c), from said first free space (21) tosaid second free space (22), or the other way round.
 39. Methodaccording to claim 38, characterized in that said at least one basket isdivided into a plurality of contiguously superimposed zones (3 a-3 c),said means (19 a, 20 a, 23) for causing at least a major portion of saidgaseous phase to flow through the basket from said first free space (21)to said second free space (22) being provided at each of said zones (3a-3 c).
 40. Method according to claims 38 and 39, characterized in thatat least one of said basket sidewalls (19,20) comprises, at a top end ofsaid at least one basket (3 a-3 c), respectively at a top end of eachzone (3 a-3 c), a gas-impermeable portion (19 a, 20 a) of predeterminedlength.
 41. Method according to any one of claims 38 to 40,characterized in that it further comprises the step of: providinggas-impermeable closure means (23) for said free spaces (21,22),disposed at said first and said second free space (21,22) of said atleast one basket (3 a-3 c), respectively of each of said zones (3 a-3c).
 42. Method according to claim 41, characterized in that it comprisesthe steps of: providing a gas-impermeable baffle (23) disposed at eithera bottom or a top end of said first free space (11), respectively ofsaid zones (3 a-3 c); providing a gas-impermeable baffle (23) disposedat either a top or a bottom end of said second free space (22),respectively of said zones (3 a-3 c).
 43. Method according to claims 31and 38, characterized in that said at least one basket (3 a-3 e)comprises a gas-impermeable cover (17) disposed at the top end thereof.44. Method according to claim 38, characterized in that it comprises thestep of: providing a plurality of baskets (3 a-3 c) for containing thefiller (4), superimposed and disposed coaxially within said column.